In previous patent applications, in particular PCT/GB96/00823 (WO 96/31790) and PCT/GB96/00367 (WO 97/04338), we have described and claimed novel techniques for spatial magnetic interrogation and novel tags. The technology described in WO 96/31790 is based on exploiting the behaviour of magnetic materials as they pass through a region of space containing a magnetic null. In particular, these earlier applications describe, inter alia, how passive tags containing one or more magnetic elements can perform as remotely-readable data carriers, the number and spatial arrangement of the elements representing information.
in the above applications we described a number of possible system embodiments employing either permanent magnets or electromagnets to create the magnetic null. We also described several system implementations some of which are particularly appropriate for tags employing very low coercivity, high permeability magnetic elements. These implementations work by detecting harmonics of a superimposed low-amplitude alternating interrogation field.
In a later application, GB9612831.9, and its successor PCT/GB97/01662, we describe arrangements which work by detecting the baseband signals generated by the passage of the tag through the magnetic null, without the need for any superimposed alternating interrogation field. A specific design for a reader in the form of a narrow slot was described in GB9612831.9.
The present application relates to magnetic readers which can be used to read data from magnetic tags operating on the principles described in WO 96/31790 and/or in WO 97/04338.
According to on aspect of the present invention, there is povided a reader for interrogating a magnetic tag, e.g. for reading data stored in the tag, which reader comprises a field generating device the magnetic field produced by which defines an interrogation zone, wherein said field generating device comprises (a) means for generating a magnetic field; (b) a transmit coil for transmitting an interrogating electromagnetic signal into the interrogation zone so as to interact with a magnetic tag, when present in said interrogation zone; and (c) at least one receive coil for receiving an electromagnetic signal generated by a tag in response to said interrogating signal and the magnetic field produced by said magnetic field generating means.
In one embodiment, the field generating device is in the form of a pair of concentric hollow cylinders; with this embodiment, the interrogation zone is defined by the interior of the inner of the two concentric cylinders.
In another embodiment, the field generating device is a flat relatively thin magnet which has poles of one polarity, e.g. north, on one face and poles of the opposite polarity, e.g. south, on the other face. With this embodiment, the interrogation zone is defined by the volume of space disposed immediately adjacent to one of the faces of the magnet, the extent of the volume in the direction perpendicular to the plane of the magnet being governed by the extent of effective interaction between themagnetic field and a tag. This second embodiment is, in effect, a single-sided readerxe2x80x94i.e. a device for reading a magnetic tag which operates when the tag passes across the face of the reader.
These two embodiments will now be described in greater detail.
The concentric hollow cylinders are preferably squatxe2x80x94i.e. the diameter of the device is relatively large compared to its length.
Preferably each of the concentric hollow cylinders supports or contains the means for generating a magnetic field; this is preferably a source of permanent magnetism, e.g. a ferrite magnet. The magnetic fields generated by such an arrangement is preferably radialxe2x80x94i.e the inner surface of a given one of the cylinders carries magnetic poles of a first polarity, e.g. north, while the outer surface of that cylinder carries magnetic poles of the opposite polarity, e.g. south. Further, it is preferred for the magnetic poles on the inner surface of the outer cylinder to correspond to those on the outer surface of the inner cylinderxe2x80x94opposed surfaces carrying poles of the same polarityxe2x80x94so that the arrangement may, for example, be S-N:N-S.
The presently preferred arrangement is to employ cylinders formed of a flexible polymer impregnated with a ferromagnetic material, e.g. ferrite. A suitable material for this purpose is xe2x80x9cFLEXOR 15xe2x80x9d manufactured and sold by Eclipse Magnetics Limited of Sheffield, England.
Preferably each of the two hollow cylinders carries both transmit and receive coils. The transmit coils are preferably wound on the cylinders in the same sense; and the receive coils are preferably wound on the two cylinders in opposite senses and connected in series.
The coils (transmit and receive) can be wound on the inner or outer surfaces of the two cylinders.
The transmit (Tx) coils are preferably positioned so that the net coupling between transmit and receive (Rx) coils is zero. This is advantageous in that it permits the use of relatively simple electronics to process the output from the receive coils and renders the system less sensitive to distortions in the transmitted waveform.
The transmit coils (one on each of the two cylinders) are generally connected in series. The windings of the inner transmit coil are preferably restricted to two zonesxe2x80x94i.e. are clumped together on two bundles, rather than being uniformly or quasi-uniformly spread out over the surface which carries them. This arrangement gives a more uniform transmitted field across the radius of the interrogation zone.
The receive coil(s) can be wound uniformly or quasi-uniformly over the surface(s) which carry(ies) them.
The contrary-winding arrangement for the two receive coils described above is advantageous because it permits an overall Rx coil arrangement which has zero net dipole moment, the opposed dipole contributions of the two component Rx coils cancelling out. This in turn gives the overall Rx coil arrangement an octopole type fall off (i.e. proportional to rxe2x88x925) sensitivity with distance away from the coil (as compared to a dipole type fall off, which is proportional to rxe2x88x923), thereby resulting in very good rejection of magnetic interference from extraneous sources (e.g. VDU screens).
Preferably the flat relatively thin magnet employed in this embodiment is in the form of a polymeric material containing permanently magnetised particles, e.g. of ferrite. For example, the material xe2x80x9cFLEXOR 15xe2x80x9d manufactured and sold by Eclipse Magnetics Limited of Sheffield, England may be used to form a thin (e.g. 0.5 mm) permanent magnet. Preferably the device includes two receive coils placed on either side of the transmit coil, all three coils being adjacent to, but spaced from, one face of a permanent magnet such as that just described. With such an arrangement, it is advantageous for the axes of the receive coils to be mutually parallel, andxe2x80x94to be perpendicular to the axis of the transmit coil. This arrangement minimises direct coupling between the transmit and receive coils. For example, the axes of the receive coils may be substantially parallel with the magnetic filed direction produced by the permanent magnet; and the axis of the transmit coil perpendicular to the magnetic field direction. This geometry is preferred because the sensitivity of the receive coils then has a quadrupole characteristic, thus allowing the reader to be relatively insensitive to interference caused by extraneous magnetic sources, e.g. VDU screens.
Where two receive coils are used, they are preferably wound in opposite senses, and connected electrically in series. This arrangement results in a zero dipole moment.
A reader in accordance with this second embodiment of the invention can have relatively small dimensionsxe2x80x94e.g. 25 mmxc3x9725 mmxc3x9710 mm; and can xe2x80x9creadxe2x80x9d data from a magnetic tag without requiring direct contact between the tag and the reader. Typically, the reader is able to function adequately over a range of several millimetresxe2x80x94e.g. with a spacing of 3-5 mm between the tag and the reader.
Readers in accordance with this embodiment are of particular use in reading data from a series of tags presented sequentially to the reader; this is of benefit, for example, in apparatus which dispenses articles carrying a magnetic label or tag for identification/stock control/pricing or other commercial purposes. One example of such apparatus is a vending machine.